1. Field of the Invention
The present invention relates to a circuit substrate and a manufacturing method thereof, and more specifically to a circuit substrate in which an interlayer connection is implemented via an interstitial via hole (hereafter referred to as IVH) and a manufacturing method thereof.
2. Description of the Related Art
In recent years, in accordance with the development of electronic devices with smaller scale and higher density, there is an increasing demand for a circuit substrate suitable for smaller scale and higher density not only for industrial use but also widely for public use. As an example of a circuit substrate devised to meet such a demand, there is one disclosed in Japanese Patent No. 2,601,128. This is a multi-layer circuit substrate consisting of IVH resin layers in which an interlayer connection is implemented via the IVH.
In this circuit substrate, wiring layers bonded on to respective surfaces of plural sheets of insulating base are in contact with a conductor such as a conductive powder filled in a through hole of the insulating base, so as to establish electrical conduction and connection between the wiring layers.
This circuit substrate is manufactured, for example, in the following manner. Upon forming a through hole in an insulating base having compressibility, this through hole is filled with a resin composition containing a conductive powder. Thereafter, upon laminating a conductor foil onto a surface of the insulating base, the insulating base and the conductor foil are heated while being compressed in a thickness direction. Subsequently, the conductor foil on the surface of the insulating base is etched to form a wiring layer, thereby completing a circuit substrate.
Generally, electronic components such as represented by semiconductors mounted on a circuit substrate generate heat at the time of use. For this reason, the circuit substrate receives a radiation heat caused by heat generation of these electronic components. Further, since this heat generation changes in accordance with conditions such as the period of time of use, the circuit substrate is used under a temperature cycle of rising and falling temperature. The thermal expansion of an insulating base is generally larger than the thermal expansion of a conductor in the IVH. On the other hand, in a multi-layer circuit substrate consisting of IVH resin layers, the bonding strength between the wiring layer and the insulating base in an adjacency of the IVH is equal to or higher than the bonding strength between the wiring layer and the thermosetting resin composition in the IVH.
In contrast, there is structurally a difference of thermal expansion coefficient between the insulating base and the conductor in the IVH. For this reason, it is inevitable that the stress caused by the difference of thermal expansion coefficient is generated in a thickness direction of the circuit substrate. However, if such a stress is generated, the stress is received by the whole wiring layer and, as a result, the stress in an adjacency of the IVH connection part cannot be alleviated in the wiring layer. For this reason, if the above-mentioned temperature cycle is repeated, it is difficult to ensure high connection reliability in a multi-layer circuit substrate.